feat: premium API keys page redesign

Visual overhaul to Stripe/Vercel quality. All functionality preserved.
- Table layout with column headers (Name | Key | Tier | Last used | Status)
- Skeleton loading state replacing spinner
- Hover-reveal revoke button (reduces visual noise)
- One-time key reveal modal with amber security warning and usage hint
- Inline copy button with 2s checkmark state
- Color-coded tier badges (free/pro/enterprise)
- Security callout with Shield icon
- Footer summary showing active count and remaining slots
- AlertDialog for revoke confirmation (was plain Dialog)
- Matches UsagePage header pattern exactly

Author: DevanshuNEU